The present invention relates to a method and apparatus for grinding a rotating workpiece and more particularly, to a method and apparatus for simultaneously grinding a workpiece with first and second grinding wheels which may be orientated at compound angles to the workpiece. The first grinding wheel engages with the workpiece at a location which is substantially diametrically opposed to the location at which the second grinding wheel engages with the workpiece and the radial grinding force imparted to the workpiece by the first grinding wheel is substantially equal and opposed to the radial grinding force imparted to the workpiece by the second grinding wheel to prevent distortion of the workpiece in a radial direction.
Known grinding machines operate on a rotating workpiece with a single grinding wheel. The grinding wheel imparts radial, tangential and axial forces to the workpiece. The radial and tangential forces tend to distort the workpiece in a radial direction relative to the axis of rotation of the workpiece. Distortion of the workpiece results in reduced component accuracy, reduced grinding rates, and may even mandate additional costly straightening procedures to remove any grinding induced distortion from the workpiece.
The distortion of the workpiece caused by the radial and tangential forces exerted by the grinding wheel on the workpiece is especially disadvantageous when long, thin, cylindrical workpieces are ground. The long, thin workpieces are particularly susceptible to distortion due to grinding forces. This is especially critical when the workpiece is formed from a relatively brittle material such as a ceramic material which can break when subjected to radial forces. The distortion effected by the grinding is particularly disadvantageous when finishing internal combustion engine valves which have a long, relatively thin stem portion and is particularly troublesome when the valves are constructed of a ceramic material. In practice, the stems of ceramic engine valves are especially sensitive to distortion in a radial direction and break if the grinding force is too great. This results in a relatively slow grinding procedure on the valve stems of internal combustion engine valves to reduce the grinding forces imparted to the ceramic engine valve and to minimize distortion thereof.
The prior art also discloses the use of dual wheel surface grinders which are capable of grinding two flat surfaces on a non-rotating workpiece. The known dual wheel surface grinders are not operable to grind cylindrical surfaces on a workpiece.
It is also currently impractical to grind elongate cylindrical ceramic workpieces such as ceramic valve on a centerless grinder due to the fact that the ceramic blanks are not normally perfectly straight or round due to the casting and sintering process which creates some distortion in the valve blanks. The distorted ceramic valve blanks can not be straightened prior to grinding and warped blanks would tend to break in a centerless grinder due to their lack of concentrricity. Also, centerless grinders using diamond grinding wheels are very costly and difficult to dress.